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    How much can the front and rear tire diameters differ?

    Hi folks. Is there any Porsche info on how closely the front and
    rear tire diameters have to match in order to prevent damage to
    the viscous torque transfer unit? It seems that the published specs
    of the officially approved tires are not exactly the same. I want this
    info for considering what range to stay within if I were to run
    other tires and sizes if I were to change wheels etc.
    thanks,
    Joe

    Re: How much can the front and rear tire diameters differ?

    Quote:
    Joe Weinstein said:
    Hi folks. Is there any Porsche info on how closely the front and
    rear tire diameters have to match in order to prevent damage to
    the viscous torque transfer unit? It seems that the published specs
    of the officially approved tires are not exactly the same. I want this
    info for considering what range to stay within if I were to run
    other tires and sizes if I were to change wheels etc.
    thanks,
    Joe



    To my knowledge, more than 5% are "deadly" for the viscous coupling and less than 3% are recommended. But don't nail me on this one.

    Re: How much can the front and rear tire diameters differ?

    Thanks for the rough guidelines, and you'll be hearing from
    my lawyer...

    Re: How much can the front and rear tire diameters differ?

    Quote:
    Joe Weinstein said:
    Hi folks. Is there any Porsche info on how closely the front and
    rear tire diameters have to match in order to prevent damage to
    the viscous torque transfer unit?



    I doubt you'll find any Porsche info on thís, because their line would have to be that, if you stick with their officially approved tire combinations, you' ll have no problems with viscous coupling life!

    Just considering the VC, the right answer to your question would probably be "How short a VC life are you prepared to accept?"

    But.........., as so often in real life, there is a big BUT!

    The big BUT is that changing relative rolling radii of front and rear tires also changes relative inputs to ABS control unit, which will then influence ABS characteristics.

    Cannot give you the gospel on what will happen first, whether you will get an error report in your display, or whether ABS will become over-sensitive and cut in much too soon, or become less sensitive and not cut in when you really want it.
    But the set-up will not be what the development engineers intended you to have. And that would not necessarily be a good thing.

    fritz

    Re: How much can the front and rear tire diameters differ?

    I forgot to thank you Fritz... That makes perfect sense.
    What about the GT2s? I've seen lots of those running differnt
    diameter fronts and rears, and they have ABS too, right?
    Are the stock front and rear GT2 tires the same diameter?

    Re: How much can the front and rear tire diameters differ?

    Hello Joe,

    The effective front/rear tire radii on the GT2 díffer more than those on the Turbo, but the GT2 has its own ABS software, which already compensates for this.

    So it's not a problem.

    fritz

     
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